Plasma polymerized thin films generated from fluorinated organic monomeric gases have been studied and characterized in the literature as good electrical insulators. The low dielectric constant of fluoropolymer thin films is a prime characteristic to fulfill the need in the integrated circuit industry for a material, with good insulating properties toward electrical charge and signal, for use in the manufacture of high density, high speed integrated circuits. The fluoropolymer thin film deposited from the C.sub.2 F.sub.4 monomer, for example, demonstrates a dielectric constant of approximately 2.7.
Fluorinated organic monomers can be either surface etching in nature or plasma polymerizing in nature. This characteristic depends on the atomic fluorine to fluorocarbon ratio, F/CF.sub.x wherein x is between 1 and 3, in the reactive plasma. In "Diagnostics and Decomposition Mechanism in Radio-Frequency Discharges of Fluorocarbons Utilized for Plasma Etching or Polymerization", Plasma Chemistry and Plasma Processing, 2, 213-231 (1982) and "Mechanism of Etching and Polymerization in Radio-Frequency Discharges of CF.sub.4 -H.sub.2, CF.sub.4 -C.sub.2 F.sub.4, C.sub.2 F.sub.6 -H.sub.2, C.sub.3 F.sub.8 H.sub.2 ", J. Appl. Phys., 54, 1284-1288 (1983), d'Agostino et al. have shown that the addition of C-H groups, hydrogen, or unsaturates such as F.sub.2 C.dbd.CF.sub.2, increases the amount of CF radicals that are formed, thus the polymer deposition rate increases. The addition of hydrogen depletes the amount of fluorine present, thereby enhancing the polymerizing character of the feed gas.
The presence of even the slightest amount of oxygen in the feed gas inhibits the formation of plasma polymerized thin films and enhances the etching characteristic of the gas. The oxygen content in the feed causes the CF.sub.x component to become C-O-F, thus there is a net increase in free fluorine, and this, along with the presence of oxygen results in high surface etch rates. Therefore, oxygen-containing fluorocarbons have not been used as thin film precursors, though fluorocarbons without oxygen have been used.
The fluorocarbons used in the industry to prepare thin films, however, suffer from a severe drawback. This drawback is the slow rate at which known fluorocarbons can be deposited by the plasma polymerization process. Retajczyk et al. in "Properties of Plasma-Deposited Films Using Ethylene and Fluoroethylenes as Starting Monomers", Materials Letters, 2, 23-26 (1983), have shown that by controlling the fluorine to carbon ratio of the starting monomer, the deposition rate can also be controlled. As the fluorine to carbon ratio of the monomer increases, the deposition rate also increases, which is demonstrated by the fact that C.sub.2 F.sub.4 deposits at approximately 100 Angstroms/minute, C.sub.2 H.sub.2 F.sub.2 at 300 Angstroms/minute, and C.sub.2 HF.sub.3 at 600 Angstroms/minute. However, what has also been shown is that while the deposition rate can be increased to a more practical rate by controlling the F/C ratio, the dielectric constant for the thin film also increases, decreasing the thin films insulation efficiency. Thus, while the deposition rate increases from 100 Angstroms/minute to 300 Angstroms/minute from C.sub.2 F.sub. 4 to C.sub.2 H.sub.2 F.sub.2, the dielectric constant increases concomitantly from 2.7 to 3.3 respectively. Therefore, what is gained in one respect is lost in another equally important respect.
There is a present need for a means for increasing the deposition rate of a high fluoropolymer thin film without a corresponding increase in the dielectric constant of the resulting film.
It is one object of the present invention, therefore, to provide a process by which a fluoropolymer thin film can be deposited at a reasonable rate while maintaining a low dielectric constant.
It is a further object of the present invention to provide a thin film deposited starting monomer by plasma polymerization, having a low dielectric constant.